Transmitting circuits



Nov. 30,1926. 1,608,487

D. G. BLATTNER TRANSMITTING CIRCUITS Filed Oct. 9. 1924 lnvenfo/i' Dar/'0 619/0770 Patented Nov. 30, 1926.

UNITED STATES PATENT OFFICE.

DAVID G B LATTNER, OF BOGOTA, NEW JERSEY, ASSIGN-OR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK,

N. Y., A CORPORATION OF NEW YORK.

TRANSMITTING CIRCUITS.

Application filed October 9, 1924. Serial No. 742,533.

This invention relates to transmitting circuits and particularly to circuits for telephone transmitters of the microphone type.

An object of the invention is to reduce the amount of noise relative to speech transmission in a circuit employing a variable impedance device, such, for example, as a microphone.

In variable resistance devices of the type having a receptacle containing granular or globular particles, such as microphone transmitters, it is well known that sudden changes in the quantity of electricity flowing through the conducting particles may cause the particles to cohere or pack, and, that when in this condition, such a device loses its microphonic property to a considerable extent. It is also known that, in such a device, the flow of electricity from one particle of the microphonic material to another is accompanied by an electric disturbance, such as is characterized by an electric are, which causes a disturbing noise in the output of the circuit. The magnitude of this noise is not materially affected by the cohering effect mentioned above, so that the ratio of useful transmission, Which is obtained through the microphonic characteristic, to noise is decreased by the cohering or packing phenomena.

The cohering or packing phenomena may be encountered for example, in connection with the use of high power amplifiers employed for impressing upon loud speaking receivers current from one or more telephone transmitters, as in a public address system. In such a system, it frequently becomes necessary to switch from one transmitter to a second transmitter, and-in so doing, the opening or closing of the highly inductive input circuit may result in setting up a sudden change in the quantity of electricity flowing and thereby cause electrical packing in the transmitter.

It has been found that the electrical packing in telephone transmitters caused in the manner described above may be reduced to an unobjectionable amount by shunting each of the microphone elements of the transmitter with a non-reactive impedance of suitable magnitude prior to its disconncction from or connection to a circuit. A suitable value of this impedance in shunt to each of the transmitter elements to reduce the electrical packing is one substantially equal to the resistance of the transmitter element itself,

The invention will be clear from the following detailed description read in connection with the accompanying drawing, the single figure of which shows the invention applied to an amplifier circuit, such as is used in a public address system.

Referring to the figure, a single stage amplifier A comprising a three-electrode space discharge tube 1 is shown. The output circuit of the tube 1 is coupled by an output transformer 2 to a loud speaking receiver R. The input circuit of tube 1 through asuitable step-up transformer 3 is adapted to be connected to a microphone transmitter T,, a transmitter T or a transmitter T by means of unitary switches 8,, S or S respectively. The transmitters T T and T are preferably of the push-pull type having two microphonic elements and a diaphragm. The diaphragm of each transmitter T T or T is connected through' conductor 12, battery 4, switch 5, adjustable resistance 6 and inductance 7 to the midpoint 8 of primary vsinding 9 of input transformer 3. The electrodes of each transmitter T,, T and T are adapted to be connected by the switches S 8, or S,, respectively, through conductors 14 and 15 to the ends of the primary Winding 9 of input. transformer 3. The non-reactive impedances 10 and 11 are arranged to be connected inshunt to the elements of transmitter T,, transmitter T or transmitter T,, by means of switches 8,, S or S respectively,

Any Suitable high quality amplifier having any number of stages may be substituted for-the particular amplifier shown in the drawing. The type of amplifier shown is quite similar to that described and claimed in the Arnold Patent No. 1,403, 17 5, dated January 17 1922. Such an amplifier is capable of producing large amplification of the signals from transmitters T T or T before impressing them upon the loud speaker R. Although only one loud speaker R is shown in the drawing, any number of loud speakers connected in parallel or in any suitable relationimay be used.

In such'a system, it may be desired to close or open the common wire 12, connected to the diaphragms of transmitter T T or T by means of switch 5 in order to connect the main battery supply a to one of the transmitter circuits or to disconnect it from that circuit. Opening the switch 5 will cause a high voltage to be momentarily built up across the switch due to the high inductance of coil 7 and transformer winding 9, which voltage may cause a suflicient increase in the quantity of electricity flowing through the particular transmitter T T or T which happens to be connected in the circuit at the time, as to cause the microphonic material in the transmitter to cohere or pack, thereby decreasing the transmission of speech or other signals, but not materially aifectmg the characteristic noise of the microphonic material as previously explained. To prevent packing due to this cause, the condenser 13 is shunted across the switch 5cand the variable resistance 6. This condenser 13 prevents sudden changes in the flow of current when switch 5 is opened. If the condenser 13 is of suitable capacity, the current flowing at the instant the switch 5 is opened will be equal to the current flowing previously and will gradually decrease to zero after a time suflicient for the charge on the condenser to build up a potential across the condenser equal to that of battery 4. In this case it is therefore clear, that no packing of the microphonic element will result.

In such a system as described, where several transmitters are used, and switches S 55 ,5 etc. are provided for connectin them to the amplifier, it follows from the line of reasoning given that opening or closing the switch associated with any of the transmitters would cause packing unless the switch 5 was opened previous to the operation of this transmitter switch. Such an arrangement would be unsatisfactory and unreliable in that it would involve the humam element. Packing can be-reduced in this case by connecting condensers of suitable capacity across each side of the switches S S and S but this arrangement is undesirable in that if two or more transmitters are provided, it

will be readily seen that the transmitter in use will deliver energy to the required load and also to the other transmitters, that part delivered to the other transmitters being wasted and the energy delivered to the receiver B being reduced by this amount. Also, as is well known in the art, the modulation of a transmitter is a function of the load impedance and is such that for a load impedance relatively small in comparison with the transmitter impedance, energy components will be produced, the frequencies of which are foreign to the driving force actuating the transmitter in use. A reduction of the load impedance is therefore undesirable.

It has been found, in accordance with the invention, that if a non-reactive impedance,

jectionable amount. It has beenfou'nd' that under the circuit conditions shown in the figure, a non-inductive resistance in shunt to each transmitter element substantially equal in magnitude to the resistance of the transmitter element itself reduces the packing to less than 2%. Changes in the re- .such as 10 or 11, is shunted,across each of sistance from this optimum value havebecn found toincrease the packing.

The invention having been generally at scribed, its nature will be more clearly brought out by the followin detailed description of its operation rea in connection with the accompanying drawing. 1

\Vith the switches and 5 in the positions shown, the primary winding 9 of the input transformer 3 is open-circuited, none of the transmitters T T or T being coupled to the amplifier A, and the impedances 10 and 11 are not shunted across the elements of any of these transmitters. Now, if it is desired to connect the transmitter T to the amplifier A, switch 5 in the common wire 12 is closed to connect one terminal of the battery 4 to the midpoint of the primary winding 9 of input transformer 3 through the adjustable resistance 6 and the inductance 7. The key 16 of switch Si is then depressed to connect contacts :17 and .18and contacts19 and 20, whereby the'irnpedances 10 and 11 are re'spectivel shunted across the elements of transmitter After these contacts have been made, ifthefkey 16 is depressed further, contacts 21 and 22 and contacts 23 and 24 will be closed'thereby connecting the elements of transmitter T to the input circuit of amplifier, A. ing the key 16 still further opens contacts 25' and 26 and contacts 27 and 28 whereby the impedances 10 and 11 are respectively removed from their shunt relationwith transmitter T and the circuit is now arranged so that signals impressed upontransmitter T are amplified by amplifier A and received in loud speaker R.

v To disconnect transmitter .T, from the amplifier circuit, the key 16 is released. Contacts 25 and 26 and contacts 27 and 28 are 11 in shunt to the electrodes of transmitter epress- T Further -movement of key 16 will open contacts 21 and 22 and contacts 23 and 24 to break the connection of transmitter '1,

through the outside wires 14 and 15 to the ends of the primary winding 9 of transformer 3. Further movement of the key 16 will open contacts 17 and-18 and contacts 19 nected in shunt with the transmitter electrodes during the interval of connecting or disconnecting the transmitter from the input circuitof the amplifier A.

In a similar manner, switches S and S or other switches connected 111 the same way,

may be arranged with contacts as shown inv switch S and may be operated in the manner described above to connect transmitter T T or any other transmitter to or disconnect it from the input circuit of amplifier A. The impedances 10 and 11 in the manner described above, will beautomatically connected in shunt with the electrodes of the particular transmitter to -be connected or disconnected during the interval of switching. As explained above, if the im'pedances 10 and 11, which are temporarily connected in shunt with the transmitter electrodesduring the interval of. switching, are of the proper magnitude, cohering or packing of the microphonic material, due to transientvoltages caused by the operation of the switches, will be reduced to an unobjectionable amount. As stated above, an optimum value for each of these impedances will be in the neighborhood of the valueof the re- IZistance of the transmitter electrode shunted y it. I ,7

If it is desired to use two or more transmitters simultaneously, as is sometimes done, the operation of the proper switches S 55,,

will connect them into the circuit in the manner described above. In such a case, the optimum value 'of the impedances 10 and 11 to prevent packing will be somewhatdifi'erent from the case of a single transmitter but if the valves used are those optimuin for a single transmitter, the packing resulting for any number of transmitters, up to 5 or 6, will not be serious.

Although the invention has been described in connection with its use in a circuit in connection with telephone transmitters, such as in a public address system, it is not limited to such use, but may be used to prevent packing in any situation where variable resistancedevices of the granular type, such as telephone transmitters, in an electric circuit are to be protected from transient volta es.

Although the invention hasbeen described and illustrated in connection with a transmitter of the type having a diaphragm and two 'microphonic elements, it is to be understood that it may be applied also to trans mitters of the type having a. diaphragm and any number of such elements. Moreover, the invention is not limited to systems using a switch and a condenser corresponding to 5 and 13 respectively. in the drawing.

What is claimed is 1. The method of reducing packing in a granular type resistance incident to circuit changes, comprising connecting across said resistance an energy consuming element during the circuit changes, and thereafter disconnecting said element from said resistance. i

2. The method of reducing packing in a granular type resistance incident to circuit. changes, comprising connecting across said resistance a non-reactive impedance then connecting said resistance to a circuit and then disconnecting :said non-reactive. impedance from across said resistance.

3. In combination, a transmission circuit, a plurality of transmitters, a non-inductive resistance, and unitary switching means for coupling one of said transmitters, to said transmission circuit at certain times, a second one of said transmitters to said transmission circuit at other times, said switching means also operating so that said resistance is maintained in shunt to the one of said. transmitters coupled to said circuit while saidcircuit is being changed from said'tran'smitter to' another transmitter.

4. In combination, a transmissiorrcircuit, a

telephone transmitter, a non-inductive resistance, and un tary switching means for performing the'followingoperations in the order'given: first, connecting said non-inductive resistance in shunt to said telephone transmitter; second, connecting said telephone transmitter to saidcircuit; third, disconnecting said non-inductive resistance from said telephone transmitter.

5. In combination, a transmission circuit, a telephone. transmitter coupled 'to said transmission circuit, a second telephone transmitten'a non-inductive resistance, and switching means for performing the following steps in the order given: first, connecting said non-inductive resistance in shunt relation to said first-mentioned telephone transmitter; second, disconnecting said first transmitter from said circuit; third, disconnecting said non-inductive resistance from its shunt relation with said first transmitter; fourth, connecting said non-inductive resistance in shunt to said second trans mitter; fifth, connecting said second transmitter to said circuit; and sixth, removing said non-inductive resistance from its shunt relation with said second transmitter.

6. In combination, a transmission circuit, a plurality of telephone transmitters adapted to he connected to said. circuit, a

non-inductive resistance common to said transmitters, and switching means having contacts individual to said transmitters and arranged to shunt said common resistance across the associated transmitter during the time that said transmitter is being connected to or disconnected from the trans- -mission circuit and to remove said common resistance from its shunt relation to the transmitter after connection of said trans mitter to said transmission circuit is effected.

In witness, I hereunto subscribe my name this 7th day of October A. D., 1924.

DAVID G. BLATTNER. 

